1. Field of the Invention
This invention relates to a dubbing system and more particularly to a dubbing system for a tape-shaped recording medium on which an amount of information signals for a given period of time are recorded in each of many recording tracks which are formed to have different azimuth angles between adjacent tracks.
2. Description of the Related Art
Apparatuses for recording and reproducing an amount of information signals for a given period of time on and from each of tracks formed on a tape-shaped recording medium include helical-scanning type home video tape recorders (hereinafter referred to as VTRs). The dubbing system of the kind using VTRs of this type is arranged as described below:
The home VTRs which are known in general are of the kind called a two-head helical scanning type. FIGS. 1(a) and l(b) show the head arrangement generally employed for the VTR of this kind. In FIGS. 1(a) and l(b), a reference numeral 1 denotes a magnetic tape. Tape guides 2a and 2b are provided for wrapping the tape 1 at least 180 degrees around the periphery of a rotary drum 3. Rotary heads HA and HB are mounted on the rotary drum 3 at the same height in the direction of the axis of rotation and, as shown, are arranged to have a phase difference of 180 degrees between them. The azimuth angle of one head differs from that of the other. As well known, these heads HA and HB are arranged to record and reproduce an amount of a video signal for one field while they rotate 180 degrees.
For the VTR of this kind, the length of track in which one field amount of video signal is to be recorded is specified as a standard. Accordingly, the diameter of the rotary drum 3 is determined by the specified length of track. This has prevented a desired reduction in size and weight of the VTR, because the drum 3 cannot be arranged to have a smaller diameter.
In view of this, VTRs which are arranged to permit a reduction in diameter of the drum have been proposed. The VTR of that kind is arranged as follows: FIGS. 2(a ) and 2(b) show the head arrangement of the VTR of the kind using a small-diameter drum. Referring to these figures, rotary heads Ha and Hb have different azimuth angles from each other. They make one turn in the one-field period of a video signal. The heads Ha and Hb are arranged close to each other to rotate with the phase difference of a very small angle .THETA..degree.. They are mounted on a rotary drum 4 at the same height in the direction of the axis of rotation.
A tape 1 is wrapped at least 300 degrees around the drum 4. One field amount of the video signal is recorded while the rotary heads Ha and Hb rotate 300 degrees respectively. In other words, this VTR is capable of recording one field amount of the video signal within a period shorter than the conventional one-field period of the video signal.
Therefore, assuming that the video signal to be recorded by the VTR of that kind is of the NTSC system, the video signal must have a vertical scanning frequency fv at 60 Hz and a horizontal scanning frequency fh at 18.9 (15.75.times.6/5) KHz instead of those frequencies fv and fh of the normal NTSC signal which are 60 Hz and 15.75 KHz. In other words, the video signal to be recorded by the VTR of that kind either must be time-base-compressed to 5/6 of the normal TV signal or must be obtained from a video camera specially designed for that VTR.
The above-stated special video camera scans a picture plane of an aspect ratio 9:10 as indicated by a broken line Y in FIG. 3 and produces a picture of an aspect ratio 3:4 as an effective picture plane as indicated by a full line X within a 5/6 field period. The heads Ha and Hb shown in FIGS. 2(a ) and 2(b) are capable of forming one track within the 5/6 field period. Therefore, one field amount of the video signal can be recorded in each track, so that the video signal is recordable by the same format as the VTR having the head arrangement as shown in FIGS. 1(a ) and 1(b). In other words, with the VTR of the head arrangement as shown in FIGS. 2(a ) and 2(b) arranged to record the above-stated video signal, the drum diameter can be reduced to 3/5 of that of the VTR of the head arrangement shown in FIGS. 1(a ) and 1(b).
Further, an arrangement to time-base-expand a reproduced video signal to 6/5 of it per field enables the VTR of the head arrangement of FIGS. 2(a ) and 2(b) to operate in the same manner as the VTR of the head arrangement shown in FIGS. 1(a ) and 1(b).
Generally, a dubbing operation on a tape is performed with two VTR units. For example, in dubbing a two-hour amount of motion picture information, the standard TV signal is transferred for a period of two hours in real time. However, as a result of popularization of the camera-integrated VTRs, many people now wish to do editing and dubbing work in an easy and simple manner. The real-time dubbing is too troublesome for users who are accustomed to high-speed dubbing with audio tape recorders. For example, deletion of a very small part of a two-hour long motion picture necessitates much time and labor.
In dubbing with the VTR having the small drum as shown in FIGS. 2(a ) and 2(b), the video signal which has been time-base-compressed to 5/6 per field is temporarily time-base-expanded to make it into the form of the standard TV signal. After that, the signal must be again time-base-compressed for every field. It thus has necessitated various processes for high-speed dubbing.
Further, in cases where the apparatus to be used for dubbing is of the kind called a four-frequency type, a tracking control circuit becomes complex and thus has hindered the realization of high-speed dubbing.